Transparent electrically-conductive layers (TCL) of metal oxides such as indium tin oxide (ITO), antimony doped tin oxide, and cadmium stannate (cadmium tin oxide) are commonly used in the manufacture of electrooptical display devices such as liquid crystal display devices (LCDs), electroluminescent display devices, photocells, solid-state image sensors, electrochromic windows and the like.
Devices such as flat panel displays, typically contain a substrate provided with an indium tin oxide (ITO) layer as a transparent electrode. The coating of ITO is carried out by vacuum sputtering methods which involve high substrate temperature conditions up to 250° C., and therefore, glass substrates are generally used. The high cost of the fabrication methods and the low flexibility of such electrodes, due to the brittleness of the inorganic ITO layer as well as the glass substrate, limit the range of potential applications. As a result, there is a growing interest in making all-organic devices, comprising plastic resins as a flexible substrate and organic electroconductive polymer layers as an electrode. Such plastic electronics allow low cost devices with new properties. Flexible plastic substrates can be provided with an electroconductive polymer layer by continuous hopper or roller coating methods (compared to batch process such as sputtering) and the resulting organic electrodes enable the “roll to roll” fabrication of electronic devices which are more flexible, lower cost, and lower weight.
Electronically conductive polymers have received attention from various industries because of their electronic conductivity. Although many of these polymers are highly colored and are less suited for TCL applications, some of these electronically conductive polymers, such as substituted or unsubstituted pyrrole-containing polymers (as mentioned in U.S. Pat. Nos. 5,665,498 and 5,674,654), substituted or unsubstituted thiophene-containing polymers (as mentioned in U.S. Pat. Nos. 5,300,575, 5,312,681, 5,354,613, 5,370,981, 5,372,924, 5,391,472, 5,403,467, 5,443,944, 5,575,898, 4,987,042, and 4,731,408) and substituted or unsubstituted aniline-containing polymers (as mentioned in U.S. Pat. Nos. 5,716,550, 5,093,439, and 4,070,189) are transparent and not prohibitively colored, at least when coated in thin layers at moderate coverage. Because of their electronic conductivity these polymers can provide excellent process-surviving, humidity independent antistatic characteristics when coated on plastic substrates used for photographic imaging applications (vide, for example, U.S. Pat. Nos. 6,096,491; 6,124,083; 6,190,846)
U.S. Pat. No. 5,300,575 describes a method for preparing polythiophene in an aqueous medium by oxidative polymerization of a suitable monomer in the presence of a polyanion. In U.S. Pat. Nos. 5,766,515 and 6,083,635, it has been disclosed that highly conductive layers of polythiophene can be obtained, when aqueous coating compositions of polythiophene and compounds containing a di- or polyhydroxy and/or a carboxyl group, amide or lactam group are coated on suitable substrates and annealed at high temperature. Although the addition of such conductivity enhancing agents (CEAs) can generate high conductivity polythiophene coatings, manufacturing of such coatings at large scale may require coating facilities with provision for handling volatile organic compounds (VOCs). Additionally, the high temperature annealing of these coatings may not be suitable for some flexible polymeric supports.
Coated layers of organic electroconductive polymers can be patterned into electrode arrays using different methods. The known wet-etching microlithography technique is described in WO97/18944 and U.S. Pat. No. 5,976,274 wherein a positive or negative photoresist is applied on top of a coated layer of an organic electroconductive polymer, and after the steps of selectively exposing the photoresist to UV light, developing the photoresist, etching the electroconductive polymer layer and finally stripping the non-developed photoresist, a patterned layer is obtained. In U.S. Pat. No. 5,561,030 a similar method is used to form the pattern except that the pattern is formed in a continuous layer of prepolymer which is not yet conductive and that after washing the mask away the remaining prepolymer is rendered conductive by oxidation. Such methods that involve conventional lithographic techniques are cumbersome as they involve many steps and require the use of hazardous chemicals.
EP-A-615 256 describes a method to produce a pattern of a conductive polymer on a substrate that involves coating and drying a composition containing 3,4-ethylenedioxythiophene monomer, an oxidation agent, and a base; exposing the dried layer to UV radiation through a mask; and then heating. The UV exposed areas of the coating comprise non-conductive polymer and the unexposed areas comprise conductive polymer. The formation of a conductive polymer pattern in accordance with this method does not require the coating and patterning of a separate photoresist layer.
U.S. Pat. No. 6,045,977 describes a process for patterning conductive polyaniline layers containing a photobase generator. UV exposure of such layers produces a base that reduces the conductivity in the exposed areas.
EP-A-1 054 414 describes a method to pattern a conductive polymer layer by printing an electrode pattern onto said conductive polymer layer using a printing solution containing an oxidant selected from the group ClO−, BrO−, MnO4−, Cr2O7−2, S2O8−2, and H2O2. The areas of the conductive layer exposed to the oxidant solution are rendered nonconductive.
Research Disclosure, November 1998, page 1473 (disclosure no. 41548) describes various means to form patterns in conducting polymer, including photoablation wherein the selected areas are removed from the substrate by laser irradiation. Such photoablation processes are convenient, dry, one-step methods but the generation of debris may require a wet cleaning step and may contaminate the optics and mechanics of the laser device. Prior art methods involving removal of the electroconductive polymer to form the electrode pattern also induce a difference of the optical density between electroconductive and non-conductive areas of the patterned surface, which should be avoided.
Methods of patterning organic electroconductive polymer layers by image-wise heating by means of a laser have been disclosed in EP 1 079 397 A1. That method induces about a 10 to 1000 fold decrease in resistivity without substantially ablating or destroying the layer.
The application of electronically conductive polymers in display related devices has been envisioned in the past. EP 0 996 599 B1 describes a light transmissive substrate having a light transmissive conductive polymer coating for use in a touch screen coating on a CRT or LCD display screen. U.S. Pat. No. 5,738,934 describes touch screen cover sheets having a conductive polymer coating.
U.S. Pat. Nos. 5,828,432 and 5,976,284 describe conductive polymer layers employed in liquid crystal display devices. The example conductive layers are highly conductive but typically have transparency of 60% or less.
Use of polythiophene as transparent field spreading layers in displays comprising polymer dispersed liquid crystals has been disclosed in U.S. Pat. Nos. 6,639,637 and 6,707,517. However, the polythiophene layers in these patents are non-conductive in nature.
Use of conductive high molecular film for preventing the fringe field in the in-plane switching mode in liquid crystal display has been proposed in U.S. Pat. No. 5,959,708. However, the conductivity requirement for these films appears to be not very stringent. For example, in one embodiment (col. 5, lines 6-10) the high molecular film can be totally non-conductive. Moreover, U.S. Pat. No. 5,959,708 does not refer to any specification involving transmission characteristics of these films.
Use of transparent coating on glass substrates for cathode ray tubes using polythiophene and silicon oxide composites has been disclosed in U.S. Pat. No. 6,404,120. However, the method suggests in-situ polymerization of an ethylenedioxythiohene monomer on glass, baking it at an elevated temperature and subsequent washing with tetra ethyl orthosilicate. Such an involved process may be difficult to practice for roll-to-roll production of a wide flexible plastic substrate.
Use of in-situ polymerized polythiophene and polypyrrole has been proposed in U.S. Pat Appl. Pub. 2003/0008135 A1 as conductive films, for ITO replacement. As mentioned earlier, such processes are difficult to implement for roll-to-roll production of conductive coatings. In the same patent application, a comparative example was created using a dispersion of poly(3,4ethylene dioxythiophene)/polystyrene sulfonic acid which resulted in inferior coating properties.
Recently, U.S. Pat. Appl. Pub. 2003/0193042 A1 claims further improvement in conductivity of polythiophene through the addition of a substantial quantity of organic compounds such as phenols. But, health and safety concerns will dictate special precautionary measures, which may need to be taken, for the introduction of such compounds to a typical web manufacturing and coating site, thus possibly adding cost to the final product.
Ionic liquids have recently received considerable attention as electrolytes in various electrochemical devices. A method for preparing electroactive conjugated polymer layer from an ionic liquid that contains a monomer of the conjugated polymer has been disclosed in U.S. Pat. No. 6,667,825 B2. U.S. Pat. No. 6,828,062 B2 discloses an electrochemical device comprising a conjugated polymer electrode, a counter electrode and an ionic liquid between the two electrodes. Thermoelectric materials comprising an organic thermoelectric component and an inorganic thermoelectric component have been disclosed in U.S. Pat. No. 6,759,587 B2. The organic thermoelectric component was suggested to include polyaniline, polypyrrole, polythiophene, or derivatives thereof. The thermoelectric material of U.S. Pat. No. 6,759,587 B2 optionally comprised ionic liquid as plasticizer. However, none of the aforementioned patents suggest the use of ionic liquid as conductivity enhancing agent for electronically conductive polymers. Moreover, the inorganic thermoelectric component of U.S. Pat. No. 6,759,587 B2 are disclosed to have a particle size up to several hundreds μm, which are unsuitable for use in any transparent layer.
As indicated herein above, the art discloses a wide variety of electrically conductive TCL compositions that can be incorporated in displays. Although application of electronically conductive polymers in display related devices has been contemplated in the past, the stringent requirement of high transparency and low surface electrical resistivity demanded by modern display devices is extremely difficult to attain with electronically conductive polymers Thus, there is still a critical need in the art for electronically conductive polymers that can be coated roll-to-roll on a wide variety of substrates under typical manufacturing conditions using environmentally desirable components. In addition to providing superior electrode performance, the TCL layers also must be highly transparent, must be patternable, and be manufacturable at a reasonable cost.